The present invention is directed to the manufacture of masks used in the lithographic production of integrated circuits and, in particular, to the manufacture of alternating phase shifting masks (altPSMs).
As an alternative to chrome on glass (COG) masks used in the lithographic production of integrated circuits, alternating phase shifting masks (altPSMs) have been employed in order to increase the resolution of the critical active area patterns projected. Such increased resolution enables smaller line widths to be exposed on the resist and consequently etched into or deposited on the wafer substrate. This is done by manipulating the electric field vector or phase of the energy beam, e.g., visible or ultraviolet light, used in the lithographic process. This phase variation is achieved in PSMs by modifying the length that a light beam travels through the mask material. By recessing the mask to an appropriate depth, light traversing the thinner portion of the mask and light traversing the thicker portion of the masks will be 180° out of phase, that is, their electric field vector will be of equal magnitude, but point in exactly the opposite direction, so that any interaction between these light beams results in perfect cancellation. The benefit of using altPSM to image narrow lines with extremely tight dimensional control has been extensively documented.
A commonly used approach to exploit the benefits of altPSM while minimizing the layout impact, is to focus the altPSM design exclusively on the most resolution and line-width-control sensitive portions of an integrated circuit chip layout. One widely used application of this approach is called gate-shrink altPSM in which altPSM segments are applied only to the imaging of the actual transistors in a poly-conductor level, while all the poly-wiring structures (e.g., polysilicon over isolation) are imaged with less layout invasive techniques, such as COG. Limiting the use of altPSM only to the poly-gate, i.e. the region of the layout defined by the polysilicon-diffusion intersection vastly simplifies the design process by limiting the target layout patterns to relatively basic rectangular shapes in well-defined proximity environments.
However, since the transition regions from altPSM to non-altPSM imaged layout portions suffer from poor image quality and dimensional control, it is normally a lithography optimization goal to apply altPSM to large, uninterrupted segments of the layout. One means of balancing the desire of applying altPSM widely to large portions of the layout, while at the same time containing the altPSM shapes to a simple local environment, is described in the SPIE publication “Alternating Phase Shifted Mask for Logic Gate Levels, Design and Mask Manufacturing” authored in part by one of the inventors of the instant application. The drawback of the iterative localization described in this publication is the enormous computational effort and resulting long runtimes involved in repeatedly deriving and checking altPSM solutions.